With the advent of 3G and 4G networks and the development of transport channels with higher modulation bandwidth and data rates, such as Long Term Evolution (LTE) transport channels, increasing the quality of the link between the base station and the mobile user device is critical to improving service. One of the most difficult problems in maintaining a quality link between the mobile user device and the base station is the constant variation of objects and fluctuating environmental conditions between the mobile user device and the base station. For example, an antenna's input impedance as seen from a base station can fluctuate chaotically as a user varies the placement of his body relative to the antenna. These problems may be particularly problematic for mobile user devices having transceivers that both transmit and receive from the same antenna. The antenna tuner in these systems must be capable of being tuned so that a receive signal can be received from the base station within a physical downlink channel and a transmission signal can be transmitted to the base station within a physical uplink channel, on the same antenna.
One prior art method of maintaining a quality link between the mobile user device and the base station is to use a directional coupler in or directly connected to the antenna tuner of the mobile user device. The direction coupler detects a mismatch between the power level of the transmission signal being received by the antenna tuner and the power level reflected back from the antenna tuner. In other words, directional couplers are utilized to measure the S11 parameter of an antenna tuner. However, optimizing the S11 parameter of the antenna tuner when both transmission signals and receive signals are being transmitted through the antenna tuner does not guarantee optimization of the power level of the transmission signal delivered at the antenna. Similarly, optimizing the S11 parameter is not directly related to and does not guarantee optimization of the power level of the receive signal at the receiver circuit of the transceiver.
Utilizing the direction coupler with the antenna tuner to determine the power level of the transmission signal also limits the ability of the transmission circuit to adjust the power level in accordance with transmission power control (TPC) information sent from the base station. As is known in the art, base stations may provide TPC information to the mobile user device to request a change in the power level of the transmission signal being provided by the mobile user device to the base station. The transmitter circuit of the transceiver in the mobile device will change the amplification provided at the transmitter circuit of the transceiver to adjust the power level of the transmission signal. Similarly, the antenna tuner may adjust its impedance to adjust the power level as requested by the base station. Unfortunately, since the directional tuner only measures the S11 parameter and both the transmission signal and the receive signal are being transmitted through the antenna tuner, the mobile device cannot detect whether the power level of the transmission signal was actually adjusted in accordance with the TPC information. Instead, the actual change of the power level of the transmission signal is not known until the transmission signal reaches the base station. This limits both the speed and the accuracy at which power changes to the transmission signal can be made.
Another problem with prior art transceivers having antenna tuners utilized with both transmission signals and receive signals is that the Q-factor of the pass band for the antenna tuner is limited by the bandwidth of the pass band. This limitation is known as the Bode-Fano limit and may be expressed for a parallel RC load impedance as:
            ∫      0      ∞        ⁢          ln      ⁢              1                                        Γ            ⁡                          (              ω              )                                                    ⁢                          ⁢              ⅆ        ω              =      π    RC  
Where ω is the angular frequency, R is the resistance value of the resistor, C is the capacitance value of the capacitor and Γ(ω) is the reflection coefficient of the antenna tuner. If we assume maximum mismatch outside of the pass band and maximum matching within the pass band, and we substitute using the known definition of the Q-factor for the pass band, the relationship between the Q-factor and bandwidth may be expressed as:
            ω      ⁢                          ⁢      c        Q    =  Δω
where ωc is the center frequency of the pass band, Δω is the 3 dB bandwidth, and Q is the Q-factor.
This expressions show that the bandwidth is inversely proportion to the Q-factor. Thus to increase the bandwidth we must decrease the Q-factor. As a result, a prior art antenna tuner in a mobile device that receives both the receive signal within a physical downlink channel and a transmission signal within a physical uplink channel has a Q-factor. Based on the expression shown above and if we assume that the reflection coefficient is substantially uniform throughout the pass band of the antenna tuner, the Q-factor of the pass band of the prior art antenna tuner is limited by the offset between the receive frequency defined by the physical downlink channel and the transmission frequency defined by the physical uplink channel. This is because this is the minimum bandwidth required to receive and transmit on the physical downlink channel and physical uplink channel. Thus, it would be desirable to increase the Q-factor toward the receive frequency and sacrifice the Q-factor at the transmission frequency, or vice versa, to increase the quality of the link between the mobile device and the antenna. Ideally however, it would be desirable to provide simultaneous matching at both the receive frequency and the transmission frequency and not have to sacrifice the Q-factor at either the receive frequency or transmission frequency.